Deposition of thin films of various materials on a substrate or wafer is one of the primary processes used in the fabrication and processing of microelectronic devices, such as integrated circuit chips. Successful film deposition generally requires control of various characteristics of the deposited films. For example, films having a predetermined uniform thickness commonly are required. Uniformity of film thickness over the surface of a wafer and from one wafer to another are similarly important. In many applications it is also important for the film to sufficiently fill vias or holes in the substrate. Finally, the deposited films must meet demanding specifications for such factors as resistivity, grain size, etc. For many years, the trend in semiconductor device fabrication has been toward ever increasing device density and the use of ever larger wafer substrates. The result of this trend has been increasingly demanding requirements for deposited films. Commercial fabrication of electronic devices having features in the sub-micron range on eight-inch wafers is now commonplace in the industry.
One of the most commonly used film deposition techniques is sputtering within a vacuum chamber using a magnetron sputtering source. In a conventional magnetron sputtering source, a low-pressure plasma consisting of positive ions from a support gas, such as argon, bombards the surface of a cathode made of a target material to be sputtered. The impact of the ions dislodges particles from the target cathode. The sputtered particles leave the target in a variety of directions and may be further scattered due to collisions with gas ions or molecules or with each other. Some of the sputtered particles are directed toward the substrate and attach to the surface of the substrate, forming a coating or film. In conventional sputtering, the deposited film is of the same material or alloy of materials as the sputter target. In reactive sputtering, a reactive gas is introduced into the vacuum chamber to form a film comprising a compound of the target material and the reactive material. For example, in a process that is becoming increasingly important in semiconductor device manufacture, material sputtered from a titanium target reacts with nitrogen which is introduced at low pressure into the sputtering chamber to form a titanium nitride film.
In sputtering sources used for semiconductor device fabrication, a spatial filter or "collimator," comprising a plurality of transmissive cells, may be positioned between the sputtering target and the substrate to prevent sputtered particles from reaching the substrate surface at low angles of incidence, (i.e., low angles between the trajectories of the particles and the surface of the substrate). Low angle sputtered material is undesirable in that it does not contribute to film growth at the bottom of holes or vias. Such particles are often deposited on the sides, near the tops of vias, thereby pinching off the amount of sputtered material that can enter the hole or via as deposition progresses. This leads to voiding within the via resulting in unacceptably poor electrical contact between layers on the substrate.
A discussion of a sputtering apparatus employing a collimating filter and of the use of collimation may be found in allowed U.S. patent application Ser. No. 07/780,882, now U.S. Pat. No. 5,330,628 coassigned herewith, the disclosure of which is incorporated by reference. Use of a collimator imparts directionality to the flux of sputtered particles reaching the surface of the substrate, thereby allowing the sputtered particles to reach the bottom of holes or vias without getting pinched off by growth on the sidewalls of the vias. Film deposited in hole and via openings grows at a rate closer to the rate of film growth on the wafer surface. As via diameters and device features have shrunk, the need for collimation when performing sputtering has increased. Applicant has shown that use of a collimator makes it is possible to fill vias less than one half micron in diameter, thereby extending the use of sputtering for via filling to at least the next generation of high density integrated circuits.
When a collimator is used, however, much of the sputtered material is deposited on the surfaces of the collimator cells. In some cases, as much as eighty percent of the sputtered material is deposited on the cell surfaces. As material is deposited on the cell walls, the cross-sectional area of the cell opening is correspondingly reduced. Even if particles are sputtered from the target cathode at a constant rate, the build-up of sputtered material on the cell walls reduces the rate at which sputtered material can pass through the collimator, thereby slowing the effective deposition rate. As the cell openings are reduced in size, the rate of build up on the collimator increases. This variation in deposition rate due to "clogging" of the collimator can lead to undesirable variations in film thickness among substrates that have undergone sputtering from the same sputtering source under otherwise equal conditions.
In most sputtering systems, the rate of deposition decreases over the life of the sputter target, further complicating the ability to obtain a film of desired uniform thickness on a wafer during a particular "run" of the system. Many prior art patents show the use of various types of deposition rate sensors which are used to monitor and adjust the deposition rate from a sputtering source so that a film of a desired thickness may be obtained. However, deposition rate sensors are expensive and unreliable and, thus, have not gained widespread commercial acceptance. In U.S. Pat. No. 4,166,783, coassigned herewith, a simple approach to compensating for the decrease in sputtering rate due to erosion of an annular target, was described and claimed. The approach of the '783 patent is to maintain a constant deposition rate by monitoring the age of the sputter target, measuring the voltage and current supplied to the plasma to determine the power dissipated in the plasma, and adjusting the plasma power according to empirically obtained information stored in a computer look-up table.
The only known prior art technique for compensating for the build up of sputtered material on a collimator used in a sputtering system involves periodically measuring the film thickness after sputtering onto a wafer to determine the effective film deposition rate, and increasing the sputtering deposition time appropriately to maintain the films reasonably close to the desired thickness. This method of maintaining uniform film thickness has many limitations. For example, it requires continuous manual intervention to effect control of sputtered film thickness, which manual intervention is time and labor intensive. Also, film thickness is subject to human error of measurement and correction. Additionally, film thickness may vary between the periodic measurements and adjustments of the sputtering source. It is noted that in semiconductor processing, as a practical matter, film deposition must be done in a single step without breaking vacuum. It is not feasible to later add to the thickness of a film which is determined to be too thin after the substrate has been removed from the sputtering system.
Accordingly, there is a need for a method for depositing a film having a uniform, predetermined thickness on a substrate using a collimated sputtering source that continuously and automatically adjusts for the build up of sputtered material on the collimator.
One object of the present invention is, therefore, to provide a method for automatically controlling a collimated sputtering source such that the thickness of film deposited by the source does not vary as sputtered material builds up on the collimator.
Another object of the present invention is to reduce the amount of manual intervention required to maintain and adjust a collimated sputtering source during deposition of thin films.
Still another object of the present invention is to reduce the variability in sputtered film thickness among a series of sputter-coated substrates by automatically compensating for the build up sputtered material on the collimator and for erosion of the target.
A further object of the present invention is to reduce the costs of production and operation related to sputtering during semiconductor device fabrication.
These and other objects of the present invention will become clear to those skilled in the art from the following detailed description, the accompanying drawings and the appended claims.